It is known that aromatic polyimides that are quite excellent in heat resistance can be obtained by polycondensation of aromatic tetracarboxylic dianhydrides with aromatic diamines ("Journal of Polymer Science" by C. E. Strong, Macromolecule Review (Vol. 11, page 161, (1976)). However, aromatic polyimides that have hitherto been generally proposed are difficult to be melt-molded and their application is limited to extruded insulating films.
To overcome the difficulty, aromatic polyetherimides obtained using aryloxy acid dianhydrides as acid dianhydride have been studied (see Japanese patent publication Nos. 20966/1982 and 20967/1982) and a polyetherimide known under the trademark "ULTEM.RTM." from General Electric Company is commercially available. Though this type of aromatic polyetherimides is excellent in melt extrusion moldability, the heat resistance thereof is lower than that of conventional aromatic polyimides.
On the other hand, other examples such as aromatic polyimides obtained by reacting aromatic diamines having a (thio)ether linkage with pyromellitic dianhydride (see Japanese Laid-Open patent application No. 170122/1984) and polyimidosulfones (see U.S. Pat. No. 4,398,021) that are not so low in heat resistance and can be melt-molded are also reported, but they are practically still unsatisfactory to be used in engineering fields and electronic parts fields where the balance of mechanical properties and heat resistance are required.